Elector beam welding apparatus chicorporating a hole center locating means

ABSTRACT

This invention relates to electron beam welding apparatus incorporating means for locating the geometrical center of a hole in a piecepart to be welded into a structure, for example the part may be a tube fitted into a hole drilled in a plate, such as a header in a heat exchanger. The apparatus provides means whereby the end of the tube inserted in the hole in the header is scanned by an electron beam of low intensity sequentially along four paths which radiate from a point outwardly and return along two mutually perpendicular axes. Means are provided whereby a pulse is generated whenever the electron beam strikes the edge of the tubing as its moves outwardly. Electronic system means are provided by which the distance along the path from the starting point of the beam is determined. Deflection means are provided which utilize the information determined to cause the beam to be deflected so that its rest position is progressively brought to the center of the tube. A memory system is provided for retaining the position and means are provided so that a weld along a circular path at a desired radius in the vicinity of the edge of the tube may be made.

United States Patent [1 1 [111 3,873,802

Sciaky Mar. 25, 1975 g ELECTOR BEAM WELDING APPARATUS CHICORPORATING AHOLE CENTER LOCATING MEANS Primary Examiner-J. V. Truhe AssistantExaminer-G. R. Peterson ABSTRACT This invention relates to electron beamwelding apparatus incorporating means for locating the geometricalcenter of a hole in a piecepart to be welded into a structure, forexample the part may be a tube titted into a hole drilled in a plate,such as a header in a heat exchanger. The apparatus provides meanswhereby the end of the tube inserted in the hole in the header isscanned by an electron beam of low intensity sequentially along tourpaths which radiate from a point outwardly and return along two mutuallyperpendicu- Iar axes. Means are provided whereby a pulse is generatedwhenever the electron beam strikes the edge of the tubing as its movesoutwardly. Electronic system means are provided by which the distancealong the path from the starting point of the beam is determined.Deflection means are provided which utilize the information determinedto cause the beam to be deflected so that its rest position isprogressively brought to the center of the tube. A memory system is [75]Inventor: Albert M. Sciaky, Palos Park, Ill.

[73] Assignee: Welding Research, Inc., Chicago, Ill.

[22] Filed: May 14, 1973 i} [2]] Appl. No.: 360,323

Related US. Application Data [60] Division of Scr. No. I66,(I(IX, July26, I97I, which is a continuation of Ser. No. 369,264, June I2, I973.Pat, No. 3,814,896.

I52] U.S.CI. ..2I9/l2l EB,2I9/I25 TP [51] Int. Cl 323k 15/00 [58]FieldofSearch ..2I9/I2IEB,I2IEM. I 2I9/I25 TP; 33/I89, I69 E:250/49.5TE. 250/495 R; 73/339 R; 324/72 EB; 318/562; 84/72 [56]References Cited UNITED STATES PATENTS 1,058,027 4/I9I3 Anthony 33/I69 C2,777,937 l/I957 Bryant i 219/125 R 3,24I,243 3/1966 Speer 33/189 X3,463,900 8/I969 Downing..... 2l9/I2l EB 3,5I3,285 5/I970 Imura 219/l2lEB radius in the vicinity 3,513,286 5/1970 Puls 2l9/I25 R made,3,783,230 I/I974 Peyrot 2I9/l2l EB FOREIGN PATENTS OR APPLICATIONS365,547 l2/I962 Switzerland 33/169 C IN POSITION, START SIGNAL providedfor retaining the position and means are provided so that a weld along acircular path at a desired of the edge of the tube may be 3 Claims, 26Drawing Figures :JTERVAL svzwma HOLE csnrznmn FINISHED SIGNAL HOLECENTERING SIGNAL w u POSTSCA," r0 SCAN/WELD INTERFACE FIG. 5 BLOCKDIAGRAM- HM CENTER LOCATOR 37 mm lam-rim 5mm.

ANTENNA SIGNAL MP. DETECTOR 27 2a 4 wt: new

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PATENTEU 3,873,802

sumaq g sAw SIGNAL TURN OFF BY SQUARE WAVE BEAM cnossEs EDGE OF WORKFROM OSCILLATOR v OUTPUT OF SEAL I I 9b PLATE CONTROL SAMPLED PART OFSIGNAL FROM ANTENNA 1 svsmu. AMP 9 9d 0 --ouTPuT 0F s5-s A 9 NORMALSIGNAL ABNORMAL SIGNAL F/6.9 ANTENNA SIGNAL AMP. OUTPUT PATENTED M8251925 SHEET u 95 g L zdumkmom wzrtwkm dank IL FM'ENTEDHARZSIQZS SHEET 5OF 8 m w G s 0 w T U P T u o W a I I I I L OUTPUT OF OPERATIONALAMPLIFIER IN SCAN CONTROL 6b I I I l l L OUTPUT OF NOR Al-H m scmCONTROL -L OUTPUT OF FF AO-L-B IN SCAN CONTROL B OUTPUT OF FF AG-L-B ll9cm CONTROL in, HU-

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sow co/vmaL SIGNALS PATEF'JTEUMREFBIQYS sumsnrs P a b w m m c d A 1 A mm D R A m A D W U A A 0 W M R R L W 0 E 4 n m w J L E S W W W l H S S yI m m m H o m m n w m F s o m M W S W T 0 M w m m m m m m T F F o w m wm m P W W T T W W A A U u 0 w S 5 0 w H u C d 9 w 7 7 7 d -01l- Ill-OQUAIRANTS I l I *llll a I 1 1 I l I l I 1 l|||| OUTPUT FROM INVERTER AMPAS-P,

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Y AXIS SIGNAL CURRENT WAVEFORM, OUTPUT OF INVERTER AMP KEEPS INPUT AT 0VLBIEL F/G. 7 SIGNALS A7 SCAN S/GNAL GENERATOR 1 ELECTOR BEAM WELDINGAPPARATUS CIIICORPORATING A HOLE CENTER LOCATING MEANS This is a divisonof application Ser. No. 166,008, filed July 26, 1971, which formedcontinuation application Ser. No. 369,264, filed June 12, 1973, now US.Pat.

This invention relates to electron beam welding and, more specifically,to a method and apparatus for joining tubes to headers of heatexchangers by the electron beam welding process. In the manufacture ofheat exchangers it is necessary to weld the ends ofa multiplicity oftubes which are inserted in a prearranged pattern of holes on a, socalled, header plate. The number of tubes and their diameter dependsupon the size of the heat exchanger and, for certain applications, asmany as 15,000 tubes having a diameter of 0.635 must be welded to theheader plate. The metals employed in the heat exchanger must have highheat resistant properties and have, in the past, been welded by thetungsten inert gas welding process/Initially, manual means were employedat a very low production rate of perhaps 200 to 300 tubes welded perweek by each operator with questionable quality inasmuch as the manualtungsten-inert gas welding process gives results which depends upon theskill and judgment of the operator. Later automatic Tig welding wasintroduced to speed the process and provide consistency of operation.This required a machine in which the Tig welding torch had to bepositioned over a tube, the wire adjusted and precaution exercised toassure a stable reproducible arc. This resulted in an increased weldingspeed and improved quality. Production by this method reached a peak ofapproximately 1,500 tubes per operator per week but was limited to thisspeed because it was necessary to manually move the welding torch fromtube to tube and accurately position it over the center of each tube inturn. The weld produced by the above method was a fillet weld whichcould be inspected only visually.

It is the object of the apparatus of this invention to produce tube toheader welds by the electron beam welding process at rates of speedheretofore unobtainable by the old methods of welding.

Another object of this invention is to position the electron beam gun bymeans of a numerical control apparatus so that it may be rapidlytransferred from one tube to the next on the header.

Another object is to determine by electronic means the amount of offsetof the gun from the geometric center of the tube to be welded and tomake a correction by deflecting the electron beam so that its new atrest position is at the geometric center of the tube.

Another object of the invention is to control the electron beam so thatit moves in a circular path at the seam between the header and the tubeat a preset speed so as to effectively form a distinct bonded areabetween the header and tube from the lip of the tube to a given distanceinto the face of the header.

Another object of the invention is to control positively the diameter ofthe circular path of the beam upon the work.

We will now refer to a particular embodiment of the invention and willrefer to the following figures to describe the method and apparatuscomprising the inventron.

FIG. 1 is a perspective drawing partially cut away showing the generalform of a heat exchanger and the various parts of the apparatusinstalled in place in accordance with the embodiment to be disclosed.

FIG. 2 is a detail in perspective of the welding gun and associatedapparatus and fixtures for moving the gun along two mutuallyperpendicular planes.

FIG. 3 is a side view of the apparatus shown in FIG. 2 with the tube andheader and parts of the sealing structure shown in cross section.

FIG. 4 illustrates a typical scanning pattern which may be utilized inpracticing the invention.

FIG. 5 is a block diagram of the electronic system for locating thecenter of the hole in the tubing.

FIGS. 6a to 6j are graphical representations of the various scan controlsignals.

FIGS. 7a to 711 are graphical representations of the signals appearingat different points of the scan signal generator.

FIG. 8 illustrates the input and output signals at the sample and holdcontrols.

FIG. 9 is a graphical representation of the signals appearing at certainsections of the seal plate detector system.

FIG. 10 is a schematic drawing of the circuit for pro cessinginformation received from the scanning system and translating it todeflection coil currents.

FIG. 1 shows the heat exchanger body 1 supported on rollers 2 so that itmay be positioned so that the holes in the header plate 3 are properlyaligned with re spect to the portable electron beam welder 4. Theportable electron beam welder is supported by the crane 5 which canposition the electron beam machine from place to place on the header.The electron beam ma chine incorporates an electron gun 6 whichgenerates the electron beam which is used to weld the end of each tube10 to the header plate 3. A diffusion pump 7 for maintaining a pressureof at most 10 Torr in the electron beam gun is mounted on the portablewelder as are two electric motors 8 and 9 which are utilized to drivethe electron beam gun upon its mounting along two axes which aremutually perpendicular to each other with respect to a seal plate 20 onFIG. 3 which seals the electron beam welding machine around a localizedarea on the header plate which contains a number of openings into whichtubes 10 are inserted and later electron beam welded. More than 350tubes may be sealed off by the seal plate.

The electron beam welding machine is shown in greater detail in FIGS. 2and 3. Referring to FIG. 2, the electron gun 6 is shown mounted to plate11 which is supported by hinges 18 on the vertical sliding member 12.The sliding member 12 may be moved in the vertical direction carryingthe electron gun with it by means of electric motor 8 and ball screw 13which drives the nut which is fixed to the slide plate 12. Slide plate12 is separated from horizontal slide plate 14 by a flexible continuoussealing means around the periphery of a vertical slot which has beenmachined in horizontal slide plate 14. The vertical slot allows theelectron beam generated in the electron gun 6 to pass from the electrongun through a hole in plates 11 and 12, through the slot in slide plate14 into the vacuum chamber 19 which is made in the space inside the headseal plate 20 and the chamber structure 21, shown on FIG. 3. The sealplate 20 is mounted to the header plate over a localized area whichcontains a number of tubes which are to be welded to the header plate.This localized area is sealed off by a seal 16 around the periphery ofthe seal plate 20. The chamber body 21 is affixed to the seal plate withsuitable flexible continuous seals between the two adjoining surfaces.The horizontal slide plate 14 may be moved by means of electric motor 9,ball screw and nut 22 which is fixed to the slide plate 14 so that whenthe screw 15 rotates the slide plate will move along in the horizontalaxis. The motion of the slide plate 14 is such that the vertical slot inthis plate may be moved from one side of the opening in chamber 21 tothe other side. Through the use of the two motors 8 and 9 and theirrespective ball screw drives the electron gun may be positioned so thatthe electron beam can be made to strike any point desired on the surfaceenclosed by the seal plate 20. By means of a suitably programmednumerical control the electron gun may be positioned step wise from onetube to another in any desired sequence for the three to four hundredtube ends 10 which are enclosed by the seal plate 20. A second vacuumchamber structure is sealed to the opposite header which, depending uponthe length of the heat exchanger, may be to 60 feet distant. This secondchamber is placed over and sealed around the opposite end of the groupof tubes which have been sealed by the electron beam welding machineseal plate. This structure is indicated at 17.

The electron beam welding machine is utilized to position the electrongun as close as possible over the center of each tube ending which it isdesired to weld. The accuracy of this positioning is never good enoughto insure that the electron beam will strike the circular seam betweenthe tube and the header at all points along the path. It is necessary,therefore, to provide a means for locating the exact geometric center ofeach tube prior to the time when it is welded to insure that theelectron beam in its rotary motion will strike the seam between the tubeand header. To this effect a method for electronically determining thecenter of the hole in the tube has been devised and is the subject ofthis application.

FIG. 5 is a block diagram illustrating the method by which the hole atthe end of the tube is located and indicates also the relationship ofseveral safety devices and detectors and their relationship to the basiclocator control. In order to locate the center of each tube the electronbeam generated by the electron beam gun is caused to scan the end of thetube and a portion of the header into which it is inserted, in aspecific manner. The scanning pattern is illustrated in FIG. 4. Theelectron gun is provided with deflection means which can deflect a beamalong two mutually perpendicular axes. The at rest or undeflectedposition of the beam as it would strike a plane which is normal to theaxis of the tube and situated at the end of the tube is indicated atpoint A on FIG. 4. By means of suitable currents fed to the X and Y axisdeflection coils the beam is made to scan the area around the end of thetube by moving it outwardly from point A a fixed distance outwardlytowards the right along the X axis and returning to A as indicated by Band C then swept along the Y axis outwardly and returning to A as shownby D and E followed by the sweep along F, G, H and I. As the electronbeam moves outwardly from point A it strikes the inner edge of the tubeat point K at which moment reflected electrons generated by the beamstriking the edge of the tube, will be picked up by an antenna as isexplained in US. Pat. No. 3,609,288 and a short duration pulse signalwill be generated. A second pulse will be generated when the beam movingoutwardly along the Y axis (D) strikes the inner edge of the tube, athird pulse will be generated when the beam strikes the edge of the tubeas it moves outwardly towards the left (path F) and a fourth pulse willbe generated when the beam strikes the edge of the tube as it movesoutwardly in the negative Y direction (path H). The analog of thedistance from A to each of the four points at which the beam firststrikes the inside edge of the tube during the sweeping operation issampled and held for each of the four points as positive and negativesignals. The positive and negative signals for each axis are summed andintegrated and the resulting signal applied to its respective X or Ydeflection circuit to bias the deflection circuit and bring the restposition of the beam to the center of the circle. As the rest positionof the beam is brought progressively toward the center of the circle thesweeping motion of the beam is continued. When the difference betweenthe positive and negative signals along each of the axes is zero thebeam is at the geometric center of the hole. The deflection coil biasingsignals are maintained at this level in-a memory circuit and theelectron beam then caused to rotate so that it traverses the seambetween the tube and the header at a sufficient power level to produce aweld along the circular path.

The above is a summary of the general action of the method oftheinvention. A more specific and complete description will now follow.

SCAN CONTROL SYSTEM The scan and quadrant signals required to cause thebeam to follow the path described above originates from an oscillatorthat provides saw and square wave output signals, The saw signal 6A isapplied to a scan control 24 on FIG. 5 and a scan signal generator 26.Through the action of logic circuits such as NORS and flip-flop circuitsin the scan control the saw output of the oscillator is processed so asto furnish four separate square wave gating signals, one for each of therespective periods that the beam is to be swept sequentially in the +X,+Y, X and -Y directions.

FIG. 6 is a graphical representation of the signals that appear in thevarious sections of the'scan control. 6A illustrates the saw toothoutput of the oscillator which is fed to an operational amplifier. Thisamplifier is connected as an inverter with a zener feedback providing anegative clamp. The input signal drives the amplifier into saturationthus providing a square wave output. The zener clamps the output atabout +10 volts. The output will swing from this level to zero volts.Since this signal is inverted, FIG. 6B, the square wave output will beof opposite polarity to the input saw 6A. The square wave output of theoperational amplifier is applied to a NOR Al-B which inverts the signalso as to obtain a square wave as shown in FIG. 6C. The output of asecond NOR AI-H shown in 6D is inverted from the output of NOR Al-B. Thesignal shown in FIG. 6D is fed to a flip-flop which changes its stateeach time its input changes from one to zero so that the signal at itsoutput is as shown in 6E. 6F illustrates the output signal from a secondflip-flop output terminal of the same.

The gating pulses for each of the four quadrants are produced in thefollowing manner.

Signal 6D and 6F are fed to NOR A2-U so as to produce an output as shownin 6G. Signals 6C and 6F are fed to the input of another NOR AZ-I-I soas to produce an output signal as shown in 6H. Signal 6D and 6E are fedto NOR A2-N resulting in an output signal as shown in 71 and signals 6Cand 6E are fed to NOR A2-B with the result that signal 6.] appears atits output. The four outputs constitute four separate gating signalswhich follow each other sequentially and are used in conjunction withthe saw output of the oscillator to produce the scanning signal whichcauses the beam to scan the area at the end of each tube in the patternas indicated above and illustrated in FIG. 4.

SCAN SIGNAL GENERATOR The scan signal generator 26 on FIG. 5 consists ofinverter amplifiers and switching modules which which function as isdescribed below so as to generate the signals that will control thedeflection of the beam along the X and Y axis so that the beam followsthe desired scanning pattern.

FIG. 7 illustrates the time relationship of the signals which appearatvarious points in the scan signal generator.

7A illustrates the saw output of the oscillator which is fed to aninverter amplifier at whose output the inverted saw tooth shown in 78appears. Four switches are utilized to separate four successive halfcycles of the saw output of the oscillator. Each of the switchesoperates in conjunction with its respective square wave gating signalgenerated in the scan control circuit for each direction or scan. A zerosignal holds the switches open, a one signal closes the switch andallows the continuous saw tooth wave to be passed. The signal ofquadrant A has been designated as being along the X plus direction,quadrant B as the Y plus direction, quadrant C as the X minus directionand quadrant D as the Y minus direction. Saw tooth 7B is passed throughswitch 2 under the control of the square wave output of NOR A2-U shownin 6G with the result that the output of switch 2 appears as shown in7C. Saw tooth 7A is applied to switch 1 which is controlled by the scancontrol signal 61 with the result that the minus X or quadrant C scansignal shown in 7E is passed. Signals 7C and 7E are then fed to anotherinverter amplifier which combines and inverts both signals to producethe output shown in 76 which is the signal which controls the X axisdeflection coil. Signals 7D and 7F are then produced by passing sawtooth 7A and 78 respectively through switches 4 and 3 respectively underthe control of square wave gating signals shown in 6H and 6Frespectively. Signals 7D and 7F are applied to an inverter amplifierwhose output has the form shown in graph 7H which is the scan signal forthe Y axis deflection coil. Signals 7G and 7H applied respectively tothe X and Y axis deflection coils will produce the scan patternillustrated in FIG. 4.

When the electron beam during its scanning motion strikes the work piecesome of the electrons are reflected and are collected by an antenna andfed to a pulse forming circuit which generates a narrow pulse whichprovides a signal effectively indicating the distance from the startpoint of the beam for that particular sweep to the leading edge of thework piece in each quadrant.

SAMPLE/HOLD AND SWITCH INTEGRATOR The sample hold circuit provides fouroutput signals that effectively indicate the radii of the scan from therest position of the beam to the leading edge of the work in eachquadrant. Each radius is represented by a DC level corresponding to theamplitude of the saw signal of the operating quadrant at the time thebeam strikes the leading edge of the work. The sample hold circuitconsists of several NOR gates and a sample hold module for eachquadrant. The sample hold modules provide an output for each quadrant.Each module has two operating modes, sample and hold, and two inputs,control and signal. A one signal from a NOR at the control inputtransfers the module to the sample mode. In this mode, the outputfollows the input. When the control input is returned to zero, theoutput is clamped at a DC level corresponding to the amplitude of thesignal at the signal input terminal. Each sample of the sample holdmodule is activated in turn during its respective scanning period orquadrant. For a particular quadrant the output of the sample hold is thevoltage of the last held sample. As the scanning beam strikes the holeedge a control pulse is formed and the leading edge of the control pulsewhich rises rapidly to a one is applied to the control terminal of thesample hold module so that it transfers to the sample mode. The outputwill follow the sampled signal and when the square wave returns from oneto zero the sample hold module is immediately placed in the holdcondition so that the amplitude of the sweep at the moment the scanningbeam struck the edge of the seam is retained in the sample hold memory.This action is repeated in each of the four sample hold modules as eachquadrant is scanned. The saw outputs of the scan signal generatorprovide the signal inputs during the time that the signals are active ineach of the particular quadrants. Each X and Y saw output from the scansignal generator has a zero to plus 10 volts and a zero to minus 10 voltcomponent. For each axis output one module only is turned on when thesaw is plus and the other when the saw is minus. Each module will thusprovide an output that ranges from zero to 10 volts with a polarity thatis the same as the input saw during the time the module is on. The exactlevel of the output signal will depend on the diameter of the hole andthe distance from the at rest position of the beam with respect to theedge of the seam for the quadrant being scanned. The polarity of the sawduring quadrants A and B is positive and during quadrants C and D isnegative. In the four sample hold modules two have outputs along the Xaxis, one positive the second negative, and two have Y axis outputs, onepositive and one negative. The positive and negative outputs of the Xmodules and the positive and negative outputs of the Y modules aresummed at associated amplifiers in a switched integrator circuit. Theswitched integrator circuits have two control terminals let us call themP and Q which act upon the four outputs of the sample hold modules inorder to produce the separate X axis and Y axis offset signals whichwill be directed to the deflection circuits of the electron beam gunsystem so as to bias the currents through the separate deflection coilsso as to bring the beam automatically to the center of the tube to bewelded. The X plus and minus output of the sample hold X axis system isfed to the switch-integrator and when the P and Q terminals are bothpositive with respect to the ground the algebraic sum of the two Xvoltages will be integrated. Likewise the algebraic sum of the plus andminus Y axis hold voltages will be integrated in a separate integrator.When a positive is applied to the P terminal and a negative voltage isapplied to the Q terminal the output of the integration will be held inmemory for each of the axes. When a negative voltage is supplied to Pand either positive or negative on Q the integrator will be reset tozero.

FIG. 8 illustrates the input and output signals at the sample hold. 8Arepresents the output of the oscillator which is running at about 160Hz. 8B represents the pulses which are formed as the beam strikes theedge of the tube in each quadrant. 8D, E, F and G illustrate the pulsesas they have been separated for each of the quadrants and 9H, 91, 9] and9K represent the four voltages present at the output of the sample holdswitches during several scanning sequences. During the scanningoperation. these voltages will change inasmuch as the beam is beingmoved towards the center of the tube as the scanning progresses.

Note therefore that as the voltage held at the output of H4, whichrepresents the X plus direction, is be- I comming morepositive, thevoltage held at the output of 8H3, representing the X minus direction,becomes less negative until the sum of the X plus and X minus voltagesapproaches Zero. When this occurs for both I the X and Y axis the beamis being held at rest at the geometric center of the circle.

HOLE RADIUS EXTRACTOR The four output voltages from the sample holdmodules are also delivered to a hole radius extractor which sums theabsolute value of these voltages and divides the sum by four so as toobtain the radius of the end of the tube being scanned. This signal isdelivered to the beam deflection circuit in order to control the radiusof the circle which is to be swept by the beam during the weldingoperation.

The above describes the method in accordance with the invention oflocating the center of a hole through the use of an electron beam andassociated circuitry and apparatus.

The overall hole center locator control includes an interval steppingfunction which controls intervals termed pre-scan, scan and post-scanand provides signals-to initiate operations associated with eachinterval,

' as well as several auxiliary functions which serve as safetys toinhibit the welding function under certain conditions. The pre-scansystem provides a signal to initiate the scan interval and areset-unlock signal to the Integrator-Switches of the X and Y axes. Thetime delay provided by this circuit allows the filament of the electronbeam gun to reach its operating temperature.

This circuit consists of several NORS and associated components.

At the beginning of the pre-scan interval the output of one of the NORSprovides a momentary zero to terminal P of the Switch-Integrators. Asecond output from another NOR switches to a 1 at the end of the prescaninterval approximately /2 second and provides an initiating signal tothe scan interval circuit which, in turn, provides an initiating signalto the Q terminal of the switch integrators for the X and Y axes and anunlock signal to the signal detector circuit. The time delay provided bythe scan circuit allows the associated circuits to locate the at restposition of the beam at the center of the hole. At the end of the scaninterval one of its output signals initiates the post-scan interval andlocks the switched integrator circuit. The post-scan circuit provides anunlock signal to the seal'plate detector circuit and ahole-centering-finished signal to the sequence circuits of the welder.The delay provided by this circuit provides time for the seal platedetector, one of the auxiliary functions provided for in the control, toperform its function. Other auxiliary functions are the no-hole detectorand the X and Y offset-limit. The hole/no-hole detector is provided toinsure that a hole is sufficiently in position so that an edge-of-workpulse is obtained in each quadrant. If these conditions are not obtainedan inhibit signal is provided to the weld sequence logic which controlsthe actual welding of the tube to the header. The X and Y offset-limitcircuit monitors the location of the hole with respect to a signalrepresenting the normal position of the beam with no bias on thedeflection coils. If this location is within prescribed limits of thecenter on both axes a qualifying signal is provided to the weld sequencelogic. If this location is beyond the preset distance at either axis aninhibit signal is provided. The seal plate control provides a signal tounlock the seal plate detector for a momentary sampling interval eachtime the beam crosses the leading edge of the work in the post-scaninterval. Should the seal plate be too close to the edge of a hole, asignal, as shown in FIG. 9E, will be generated which indicates that thehole is out of position with respect to the seal plate. In this casethis signal will cause an inhibit signal to be delivered to the weldsequence logic circuits. When the hole is correctly positioned withrespect to the seal plate, the beam does not strike the seal plate and aqualifying signal is sent to the weld sequence logic circuit which willallow the welding to proceed if the proper qualifying signals areobtained from the X and Y offset limit control and the hole/no-holedetector.

FIG. 9 illustrates the signals which will be present in the seal platedetector system for the case where the hole is correctly positioned withrespect to the seal plate and a second case where the hole is too closeto the seal plate.

FIG. 9A illustrates the saw signal which causes the beam to deflect.

FIG. 9B is a square wave generated from the time the beam crosses theedge of the work to the time when the beam reaches its farthest positionfrom the center. If the seal plate has not been struck, the electronflow to the antenna will take the form shown in C. Should the seal platebe too close to the edge of the hole, the reflected electron currentflow will take the form shown in FIG. D. The current will reach amaximum value during the period of the square wave generated at theoutput of the seal plate control and will then dip to a lower valuebefore the beam reaches the end of its sweep. The reflected electroncurrent is differentiated and if a dip occurs in the signal, as shown inD, a positive output voltage is obtained which will cause the weldingcircuit to be inhibited.

FIG. 10 is a schematic drawing which indicates the manner in which the Xand Y axis scanning signals which cause the beam to follow the circularpath desired, the radius signal, the X and Y'axis offset signals, and asignal which is proportional to the square root of the acceleratingvoltage of the electron gun, are combined and processed in order toproduce the X axis and Y axis deflection coil currents. The X axisoffset signal is multiplied by an analogvoltage which is proportional tothe square root of the acceleration voltage of the electron gun in anelectronic multiplier whose output is fed to one of the two inputterminals of an amplifier whose output feeds the deflection amplifierwhich feeds the X axis deflection coil.

The second input terminal of this amplifier receives the X axis scanningsignal so that the output of this amplifier represents the combinedsignals for driving the X axis deflection amplifier and deflection coil.

The Y axis offset signal and scanning signal are processed in a likemanner and the currents through the X and Y axis deflection coils causethe beam to follow a circular path of the desired radius with suitablecorrection to the currents for variations in acceleration voltage.

The system may be adapted to find the center of symetrical shapes otherthan circles, for example the geometric center of squares, ellipses,hexagons and other shapes may be easily found by the method disclosed.

The method may also be applied to find the center of tubes into whichplugshave been inserted so as to seal material inside the tubes. Anexample of this would be the end closures of tubes used as fuel elementsfor nuclear reactors. The plugs must be welded to the tubes at the seamformed between tube and plug at the ends of the tubes.

In welding the plug into the tube the electron beam is directed towardsthe seam in the axial direction and is caused to follow the seam as hasbeen described above so as to produce a continuous endless weld alongthe joint between tube and plug.

The ends of the plug and tube assembly may be scanned in the same manneras the hole area is scanned in and around the tubes in the tube andheader assembly described above.

Having described the method of the invention, together with anillustrative embodiment of apparatus for carrying out the method in thewelding of tubes to headers for heat exchangers, it is understood thatthe invention is not limited to this application since it may be appliedto other shapes and forms without departing from the scope of theinvention.

What is claimed is:

1. Apparatus for welding the ends of tubes along their line ofadjoinment to the header plate of a heat exchanger which has beendrilled with holes through its thickness and into which the ends oftubes to be welded to said header are inserted, comprising:

a chamber, capable of being evacuated, which is formed with two opensides opposite one another;

sliding carriage means mounted over one of said open sides;

an electron beam welding gun comprising means for generating, focusingand deflecting an electron beam mounted on the said carriage;

means for translating the said carriage along two mutually perpendicularlines which are parallel to a plane passing through the borders of thesaid open side;

means for attaching the said chamber to the said header plate so thatthe second open side of the said chamber is covered by a portion of asurface of the said header plate which includes several of the said tubeends so that the space inside the said chamber and tubes may beevacuated;

means for sealing off the said tubes at their ends opposite the headerplate to which they are to be welded;

means for directing the aforementioned electron beam essentially normalto a plane defined by the intersection of the edge of the hole in theheader with the outer surface of the tube to be welded thereto so thatat its rest position it is within an an area in the plane circumscribedby the said edge of the hole;

means for deflecting the said beam out from and back to the at restposition ofthe beam sequentially along four separate radial paths alongmutually perpendicular axes lying on said plane;

means for generating an electrical pulse whenever the said beam strikesthe inner edge of the said tube during each of said outward deflection;

means for generating separate voltages by the action of each of saidpulses, which represent the distance and direction from the restposition of the beam to the inner edge of the said tube along each ofthe said paths;

electrical means for adding algebraically the voltages representing thesaid distances and directions on one of said axes and simultaneouslyadding alegebraically the voltages representing the said distances anddirection on the other axis so as to obtain separate voltages whichrepresent the deviation of the at rest position of the beam from thecenter of the tube along each of the said axes;

and means for causing the beam to be deflected along the said axesthrough the action of the last mentioned voltages so that the beampasses through the geometric center of the said tube.

2. Apparatus as in claim 1 including electrical means for addingarithmetically the voltages representing the distances from the saidrest position to the inner edge of the said tube for each of the saidradial paths;

electrical means for dividing the sum of said voltages by the number ofsaid paths so as to obtain a voltage representing the average value ofthe inner radius of the said tube;

electrical means for adding to said last mentioned voltage a voltagewhich represents the distance from the periphery of the hole to theaforementioned line along which the aforesaid parts are to be welded;

means for applying the resulting sum voltage to a means for controllingthe movement of the elec-.

tron beam; and means for controlling the motion of the beam so that itfollows the said line at a preset tangential velocity. 3. Apparatus asin claim 2 including means for increasing the intensity of the said beamso that a weld is effected between the aforementioned parts along thesaid line of adjoinment.

* l =l l=

1. Apparatus for welding the ends of tubes along their line ofadjoinment to the header plate of a heat exchanger which has beendrilled with holes through its thickness and into which the ends oftubes to be welded to said header are inserted, comprising: a chamber,capable of being evacuated, which is formed with two open sides oppositeone another; sliding carriage means mounted over one of said open sides;an electron beam welding gun comprising means for generating, focusingand deflecting an electron beam mounted on the said carriage; means fortranslating the said carriage along two mutually perpendicular lineswhich are parallel to a plane passing through the borders of the saidopen side; means for attaching the said chamber to the said header plateso that the second open side of the said chamber is covered by a portionof a surface of the said header plate which includes several of the saidtube ends so that the space inside the said chamber and tubes may beevacuated; means for sealing off the said tubes at their ends oppositethe header plate to which they are to be welded; means for directing theaforementioned electron beam essentially normal to a plane defined bythe intersection of the edge of the hole in the header with the outersurface of the tube to be welded thereto so that at its rest position itis within an an area in the plane circumscribed by the said edge of thehole; means for deflecting the said beam out from and back to the atrest position of the beam sequentially along four separate radial pathsalong mutually perpendicular axes lying on said plane; means forgenerating an electrical pulse whenever the said beam strikes the inneredge of the said tube during each of said outward deflection; means forgenerating separate voltages by the action of each of said pulses, whichrepresent the distance and direction from the rest position of the beamto the inner edge of the said tube along each of the said paths;electrical means for adding algebraically the voltages representing thesaid distances and directions on one of said axes and simultaneouslyadding alegebraically the voltages representing the said distances anddirection on the other axis so as to obtain separate voltages whichrepresent the deviation of the at rest position of the beam from thecenter of the tube along each of the said axes; and means for causingthe beam to be deflected along the said axes through the action of thelast mentioned voltages so that the beam passes through the geometriccenter of the said tube.
 2. Apparatus as in claim 1 including electricalmeans for adding arithmetically the voltages representing the distancesfrom the said rest position to the inner edge of the said tube for eachof the said radial paths; electrical means for dividing the sum of saidvoltages by the number of said paths so as to obtain a voltagerepresenting the average value of the inner radius of the said tube;electrical means for adding to said last mentioned voltage a voltagewhich represents the distance from the periphery of the hole to theaforementioned line along which the aforesaid parts are to be welded;means for applying the resulting sum voltage to a means for controllingthe movement of the electron beam; and means for controlling the motionof the beam so that it follows the said line at a preset tangentialvelocity.
 3. Apparatus as in claim 2 including means for increasing theintensity of the said beam so that a weld is effected between theaforementioned parts along the said line of adjoinment.